The correct answer is D. no article.
A pacifist is somebody who is against war. The word “pacifist” is a noun, and nouns do not usually take articles. In this case, the word “war” is also a noun, so it does not need an article either.
The other options are incorrect because they are all articles. The article “a” is used before singular nouns that refer to a specific person or thing. The article “an” is used before singular nouns that begin with a vowel sound. The article “the” is used before singular nouns that refer to a specific person or thing that has already been mentioned.
In this case, the word “war” is a general noun that refers to any war. It is not referring to a specific war, so it does not need an article.